Handle 202 Accepted response in HTTP client

Per the Streamable HTTP spec, a server returns 202 Accepted with no
body as the ACK for a JSON-RPC notification or response. The client
previously errored on 202 because it fell through Content-Type
dispatch. Short-circuit to return nil before Content-Type handling so
fire-and-forget messages (e.g. notifications/initialized) no longer
raise.

https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#sending-messages-to-the-server

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

test/mcp/client/http_test.rb

@@ -369,6 +369,21 @@ module MCP
         assert_equal("Invalid request", response.dig("error", "message"))
       end
 
+      def test_send_request_returns_nil_for_202_accepted_response
+        request = {
+          jsonrpc: "2.0",
+          method: "notifications/initialized",
+        }
+
+        stub_request(:post, url)
+          .with(body: request.to_json)
+          .to_return(status: 202, body: "")
+
+        response = client.send_request(request: request)
+
+        assert_nil(response)
+      end
+
       def test_send_request_raises_error_for_sse_without_response
         request = {
           jsonrpc: "2.0",